Growing interstellar cyanopolyacetyleneswith ion/molecule reactions
DIETHARD
K. BOHME,STANISLAW
WLODEK,A N D ASIT B. RAKSIT
Department of Chemistry and Centre for Research in Experimental Space Science, York Universiw,
Dowr~sview,Ont., Canada M3J l P 3
Received December 15, 1986
DIETHARD
K. BOHME,
STANISLAW
WLODEK,
and ASITB. RAKSIT.
Can. J. Chem. 65, 2057 (1987).
New and recent results of laboratory measurements are presented which allow an assessment of four ion/molecule reaction
schemes which have been proposed for the gas-phase synthesis of interstellar cyanopolyacetylenes. The schemes are scrutinized
in terms of the manner of growth of the carbon backbone and the manner in which nitrogen is entrained in the formation of ions of
the type H2Cz,1+
I N + which may neutralize by proton transfer or electron-ion recombination to form cyanopolyacetylenes.
DIETHARD
K. BOHME,
STANISLAW
WLODEK
et ASIT B. RAKSIT.
Can. J. Chem. 65, 2057 (1987).
On rapporte des r6sultats nouveaux et recents relatifs a des mesures de laboratoires qui permettent d'evaluer quatre schCmas
rCactionnels ions/molCcules qui ont Ctt proposes pour la synthkse en phase gazeuse de cyanopolyac6tyli?nes interstellaires.
On a examine les divers schCmas en fonction de la croissance de la chaine carbonee ainsi que de la f a ~ o npar laquelle l'azote
est entrain6 dans la formation d'ions du type H2C2,1+ qui peuvent Ctre neutralisks par un transfert de proton ou par une
recombinaison ionlelectron conduisant a des cyanopolyacCtyli?nes.
[Traduit par la revue]
Introduction
Perhaps the most remarkable aspect of the known chemical
composition of the interstellar medium is the abundance of
the homologous series of cyanopolyacetylene molecules which
have been identified by radioastronomers (1). The higher
members of this family of chain-like carbon molecules with
terminal cyanogen (CN) substituents are not found naturally
on earth, but their rotational transitions have been detected at
radio frequencies in all types of molecular clouds in interstellar
space as well as in circumstellar envelopes. There is now
a need to understand the mechanisms of their formation in
these environments. Several synthetic pathways have been
suggested. These include the fragmentation of grains or very
large molecules (2), catalytic surface reactions (3), and homogeneous gas-phase reactions between ions and molecules. Here
we examine the various schemes which have been proposed for
the latter pathway of gas-phase synthesis by ion chemistry,
many elements of which have been included in detailed
modelling studies of interstellar clouds. New and recent results
of laboratory measurements are presented which advance our
understanding of several of these schemes, allow their assessment, and permit some discrimination between them.
Four ion/molecule reaction schemes will be considered. In
all of them formation of the cyanopolyacetylene is achieved
ultimately by the neutralization of H2C2rl+,N+ through the
electron-ion recombination reaction [I].
,
iI
i
or by a proton-transfer reaction with a molecule having a proton
affinity higher than the cyanopolyacetylene. The schemes differ
in the manner of growth of the carbon backbone and the manner
in which nitrogen is entrained in the formation of H2C2!,+
The earliest models put forward by Winnewisser and Walmsley (4) and by Schiff and Bohme (5) allow for the buildup of the
cyanopolyacetylenes in two ways. In one scheme (Scheme A)
the acetylenic chain is conceived to grow two carbon atoms at a
time by reactions between acetylene and its ions and then to
be substituted by reaction with HCN or CN as indicated in
reactions [2] and [3].
[2]
C2,,H:3
+ HCN+
H2C2n+1N++ H,H2
Alternatively (Scheme B), substitution is achieved first, followed by growth of the carbon backbone by reactions with
acetylene ions as in reaction [4].
Growth by one carbon atom at a time is the distinguishing
feature of the scheme proposed by Woods (6) (Scheme C). The
two essential steps in this scheme are given in reactions [5]
and [6].
Finally, several authors (7-9) recently have postulated a
mechanism (Scheme D) in which build-up of the carbon chain is
followed by reactions with N atoms as in reactions [7] and [lo].
Depending on the degree of hydrogenation of the carbonaceous
reactant ion, secondary hydrogenation reactions may be required
as in reactions [8] and [9] before neutralization can yield the
cyanopolyacetylene.
Reactions with N atoms are attractive in the chemistry of dark
interstellar clouds because of the relatively high abundance of
these atoms.
Experimental
Several new reactions were measured as part of this more general
evaluation of the gas-phase synthesis of cyanopolyacetylenes by ion
chemistry. The measurements were performed with the Selected-Ion
Flow Tube (SIFT) apparatus in the Ion Chemistry Laboratory (lo, 11).
Ions were generated in an axial electron impact ionizer (Extranuclear
Model 0413) at ca. 50 eV from the pure parent gas or the parent
gas diluted with helium: C + (cyanogen), C3+(methylacetylene), Cs+
(n-pentane), C4H2+(diacetylene), and C4N+ (ally1 cyanide). The ions
were injected into helium carrier gas at ca. 10 to 50 V. The total
pressure was in the range from 0.30 to 0.35 Torr and the ambient
temperature was 296 & 2 K. The reagent gases and helium carrier gas
were of high purity ( 2 9 9 . 5 mol%). Hydrogen cyanide was prepared
according to the procedure described by Glemser (12). Rate constants
and product distributions were derived in the usual manner (13, 14).
2058
CAN. J. CHEM. VOL. 65. 1987
Results and discussion
In this section we provide a detailed and comprehensive
assessment of the four ion/molecule reaction schemes which
have been advanced for cyanopolyacetylene synthesis. Our
new results together with recent results are integrated into the
discussion as appropriate. An attempt is made throughout to
project the chemistry to higher members of the homologous
series for which new measurements are not available.
Series A
Scheme A rests on the likelihood of growing polyacetylene
cations. Two obvious routes for the growth of polyacetylene
cations involve the sequential addition of C2units either directly
to the ion as follows:
or indirectly by repeated reaction with the polyacetylene formed
through the neutralization of the polyacetylene cations as
follows:
~
2
~
2
3 ~ 4 ~ t . 3
~
6
~
t
3 ~ 8 ~ : , 3
Both schemes may also operate with polyatomic carbon units
containing an even number of carbon atoms, such as neutral or
ionized C4 Or C6 units. At the low pressures of the interstellar
medium the addition of the carbon unit in each step may occur
either by radiative association or by condensation as indicated in
reactions [13] to [16] for diatomic carbon units:
Only a very few of these reactions have actually been explored
in the laboratory. The available measurements provide insight
primarily into the kinetics of the initiation steps.
There is no information on the reactions of the acetylenic ions
with the molecules C2 and C2H, but it is well known that the ions
C2H2+ and C2H3+ react with acetylene to etablish C4H2+and
C4H3+ (15). Figure 1 displays results of SIFT measurements
which show how the reaction of C2H2+ with C2H2establishes
C ~ H , +and c ~ H ~as+well as the adduct ion C4H4+.The adduct
is likely to be collisionally stabilized at the pressures of the
SIFT experiments; it is not observed at low pressures in ICR
experiments (15). In contrast, the ions C4H2+and C4H3+ form
adduct ions with acetylene under SIFT conditions (16) as well as
at the low pressures of the ICR technique (15 , 17). There has
been one report of a minor channel at low pressures which leads
to C6H3+ from C4H2+ and C2H2 (15). The observation of
adducts at low pressures implies the occurrence of radiative
association, although both collisional and radiative association
have been advanced as the mechanism for the association of
C4H2+ and C4H3+ with C2H2at low pressures (15, 17).
The reactions of acetylene with C2H2+ and C2H3+ also
initiate the reaction sequence [12] and it is known that C2H+
reacts rapidly with C2H2 to produce C4H2+ exclusively (15).
SIFT measurements have shown that the chain-lengthening
reaction of diacetylene with C2H2+ to form C6H3+ must
FIG. 1 . Observations of the sequential acetylene chemistry initiated
by C 2 ~ 2in+ the reaction region of a SIFT apparatus. Three steps are
seen to lead to the connection of four C 2 units with overall complete
retention of acetylenic hydrogen ( C s H s f )and with loss of H ( C g H , ' )
and H 2 ( C ~ H ~ 'The
) : buffer gas is helium. P = 0.34Torr, 0 = 7.4 X
lo3 cm s-', L = 46 cm, and T = 296 K. The CzH2+ is derived from
acetylene by electron impact at 20 eV.
compete with a predominant charge transfer channel (16).
This may also be the case for reactions of C2H2+ with higher
polyacetylenes as the ionization energy decreases with chain
length. For the reactions with C2H3+ proton transfer will be
the competitive channel since the proton affinity of C2H2 is
expected to be lower than that of the polyacetylenes.
Some information is also available for reactions between
neutral and ionic C4 units. Recent SIFT measurements have
indicated that the reactions of C4H2+ and C4H3+ with diacetylene can establish C6H2+and C6H3+but that they are dominated
by adduct formation, as are the reactions of C6H2+ and C6H3+
with diacetylene (16). The situation is different at lower
pressures where collisional stabilization is less influential. For
the reaction of C4H2+with C4H2recent ICR measurements have
indicated the formation of the products C6H2+ (83%), C8H2+
(17%), and C8H3+ (1%) (18). Also the reaction of C4H3+ with
acetylene was observed to produce C6H3+ exclusively (18).
Other ICR measurements have shown that the C6H2+ produced
from C4H2+and C4H2reacts with diacetylene to produce C8H2+
and CloH2+ (19). Finally, it has also been shown that C4H+
reacts with diacetylene to produce
exclusively (18).
In summary, the results of laboratory measurements available
at this time indicate that polyacetylenic ions are readily
generated by sequential reactions involving C2or C4 units. SIFT
measurements suggest that collisional association reactions
begin to predominate at moderate pressures as the ionic
complexity increases, and ICR measurements indicate that ionic
2059
BOHME ET AL.
growth at the low pressures of interstellar environments may
proceed by bimolecular condensation reactions, or possibly
radiative association reactions.
For Scheme A to succeed in the ultimate formation of
cyanopolyacetylenes, the growth of polyacetylene cations must
be followed by the entrainment of CN according to reactions of
type [2] and [3]. Experimental observations are unavailable for
reactions with CN, but some measurements have been reported
for reactions with HCN. ICR measurements (15) indicate a fast
reaction of C2H2+ with HCN to produce two ions as indicated
in reaction [17]:
[17] C2H2+ + HCN
0.65
H2CN+ + C2H
Both product ions have also been observed in SIFT experiments
at a branching ratio of 0.05 each, but the predominant product
observed at the higher pressures of the SIFT was the ionic
adduct (5). C2H3+ was observed with both techniques to react
with HCN by proton transfer (1 1, 15). Recent ICR measurements indicate that both C4H2+ and C4H3+do not react rapidly
with HCN at low pressures, k < 2 x lo-'' cm3molecule-' s-'
(15). SIFT experiments carried out in this study indicate rapid
association in helium at a total pressure of 0.335 Torr and a
helium density of 1.1 x 1016 atoms cmP3 at 296 k 2 K.
Reaction [18] was observed to have an effective bimolecular
1181 C4H2+ + HCN
+ He -+
H3C5N+ + He
rate constant of 2.2 X 10-lo cm3 molecule-' s-' under these
conditions. The structure of the adduct ion is uncertain, but
it may be a source for neutral cyanodiacetylene if it loses
molecular hydrogen upon recombination with electrons. In the
interstellar environment the association must proceed radiatively.
The upper limit to the rate constant for this reaction determined
with the ICR method does not preclude this possibility, and the
high rate of collisional association certainly is suggestive of a
relatively efficient radiative association.
Scheme B
Scheme B relies on the build-up of the cyanopolyacetylenes
by their repeated reaction with the acetylenic ions C2H2+ and
C2H3+ in which the CN substituent is retained. While experiments have shown that this is achieved for one of the first
members of the family of reactions, viz. the reaction of HCN
with C2H2+,it is not for the next higher member. As indicated
earlier, ICR and SIFT measurements have shown that HCN
reacts with C2H2+ to produce some H2C3N+ while the C2H3+
ion reacts with HCN only by proton transfer. In contrast, recent
SIFT experiments in our laboratory indicate that elimination of
HCN and association dominate in the reaction of C2H2+ with
HC3N for which k = 3.2 x lop9 cm3 molecule-' s-' (20):
[19] C2H2+ + HC3N
0.5
Scheme C
The scheme proposed by Woods has as its foundation the
atomic carbon ion. Growth is achieved by adding single carbon
units through sequential reactions with C+ and methane. No
laboratory data were available for the reactions [5] and [6]
involving cyanoacetylenes when this scheme was originally
conceived by Woods. However, recent SIFT measurements in
our laboratory have shown that this scheme is indeed credible.
We have shown that C + reacts with the first member of the
series of cyanopolyacetylenes extremely rapidly, k = 6.1 x
lo-' cm3 molecule-' s-', to produce the C4N+ cation 20% of
the time as shown in reaction [20] (20):
C4H2+ + HCN
A similar result has been reported recently by Knight et al. (21)
who suggest that the ionic adduct which must be formed by
radiative association at the low pressures of interstellar clouds
may yield some HC5N by electron/ion recombination. C2H3+
is known to react with cyanoacetylene only by proton transfer
(20, 21).
We report here the results for the subsequent reaction of C4N+
with CH4 which establishes some H2C5N+. The latter ion may
neutralize to form cyanodiacetylene.
With C + as the source ion for C4N+, sufficient cyanoacetylene was added into the flow tube of the SIFT apparatus to
deplete most of the C + . The major ion in the flow tube under
these conditions was C3H+ but addition of methane did provide
an assessment of the products of the C4N+ reaction since the
products of the reaction of C3H+ with methane had been
characterized previously in our laboratory (10). Experiments
performed at low resolution identified product ions at m/z =
51 52 (C4H3+ or C3HN+ and C4H4+ or C 3 ~ 2 N + 27
),
28
(C2H3+ or CHN+ and C2H4+ or CH2N+), and 76 (C5H2N+).
The rate constant for the deca of C4N+ at 296 2 2 K was
determined to be 5.5 X lo-' cm3 molecule-' s-'. Further
experiments were performed to elucidate the product yields by
generating C4N+ from ally1 cyanide by electron impact (22).
The observed product ions with their relative yields were as
follows:
+
+
B
0.30
C4H3+ + CHN
The overall rate constant in this case was 5.7 X 10- l o cm3molecule-' s-'. Experiments with CD4 excluded C4H4+, CHN+,
and c 2 ~ 4as+possible product ions. C 3 ~ was
~ +shown to
contribute less than 5% to the total product ions. The results of
one experiment with CD4 are shown in Fig. 2.
Two pairs of the product ions in reaction [21] are related
by intramolecular proton transfer. Thus neutral and protonated
cyanoacetylene (or isocyanoacetylene) appear to contribute
60% to the total product ions, while neutral and protonated
diacetylene appear to contribute 35%. The remaining 5% of the
reaction appears to lead to protonated cyanodiacetylene or an
isomer of cyanodiacetylene.
Scheme C presumes similar reactions of C+ with the higher
members of the homologous series of cyanopolyacetylenes and
similar reactions of the derivative C2,+2N+ ions with CH4
as indicated by reactions [5] and [6]. Figure 3 projects the
2060
CAN. J. CHEM. VOL. 65. 1987
These reactions involve radicals as neutral reactants and would
be much more difficult to measure in the laboratory.
Scheme D
The hydrocarbon ions required for reactions [7] and [lo]
contain an odd number of carbon atoms. There are a number of
possible sources for C2n+lH+ ions in the interstellar environment. These include the protonation of odd carbon clusters as
illustrated by reaction [24], the hydrogenation of odd carbon ion
clusters as indicated by reaction [25], or the reactions of C+
with polyacetylenes as in reaction [26]. Reactions of type [24]
10
0
1
2
3
4
C D4 FLOW / (molecules s-l
5
x
6
loi7)
FIG. 2. The variations in ion signals observed for the addition of
deuterated methane into the reaction region of the SIFT apparatus
in which C4N+ is initially established in helium buffer gas. P =
0.35 Tom, B = 6.7 x lo3 cm s-I, L = 46 cm, and T = 294 K. The
C4N+ is derived from ally1 cyanide by electron impact at 55 eV.
have not yet been observed in the laboratory. Also, the proton
affinities of neutral carbon clusters are not known. Nevertheless, these reactions are expected to occur in interstellar clouds
because of the presence of likely proton donors such as H3+,
N2H+, and CHO+. Reactions of type [25] have recently been
investigated with the S I F I technique. For the reaction of C3+ a
rate constant of 3.0 X lo-'' cmP3 molecule-' s-' has been
reported (23). We can report here new results for our observations of the following two deuteration reactions:
Reactions [27] and [28] were found to have room temperature
rate constants of (1 .3 + 0.4) and (5 + 2) X 10- cm3 molecule- s-', respectively. Reactions of type [26], important in
interstellar clouds because of the relatively high abundance of
C+ ions, have been observed with acetylene and diacetylene for
which rateconstants of 2.2 and 1.5 X lop9 cm3molecule-' s-',
respectively, have been reported (16, 24).
Still another source of C,,H+ ions has become apparent in a
recent study of the chemistry of C4N+. This latter ion was
observed to react with acetylene and diacetylene to produce
C5H+ and C7H+ respectively through reactions of type [29]
(22):
''
'
f
.t
+
"2.
C2nH2
FIG. 3. Hypothetical scheme for the synthesis of cyanopolyacetylenes.
chemistry established in this study for the production of
cyanodiacetylene to the production of higher cyanopolyacetylenes. The scheme provides a plausible route to the synthesis of
the higher cyanopolyacetylenes. An important feature of this
scheme is the chemical feedback of the lower cyanopolyacetylene(s) which may be formed in the reactions of C2,,+2N+ with
CH4. In the interstellar chemistry the latter reactions must
compete with those involving HZ. In a separate study we have
shown that C4N+ reacts with Hz only slowly, k = 2.2 x
lo-' cm3 molecule-I S-' at 296 K, to produce C3H' (22).
Other reactions of C2,,+ 2N+ which may lead to growth of
cyanopolyacetylenes in interstellar chemistry are also possible
as is indicated, for example, by reactions [22] and [23]:
'
[22] C2,+2N+ + CH3 + H2C2,+3N+ + H
[23] C2,+2N+
+ CH? + H2C2,,+3N+ + hv
[29] C4N+
+ C2,+2H2
+
C2,,+sH+ + HCN
Possible direct sources of C2,,+,H3+ ions which are likely
in dense interstellar clouds involve reactions of CH3+ with
acetylene and polyacetylenes. The kinetics of the reactions with
acetylene and diacetylene have now been characterized in the
laboratory. The reaction with acetylene produces C3H3+ exclusively according to reaction [30] (5):
while the reaction with diacetylene produces some C5H3+
according to reaction [3 11 (16).
There is little experimental information on the reactions of
C2, + I H+ and CZn+ I H3+ with N atoms and on the subsequent
ions. Only one member of
hydrogenation of the C2,,+
reactions [7] and [lQ]has been studied in the laboratory. Recent
flow-drift experiments have provided a rate constant of 1.3 x
lo-'' cm-3 molecule-' s-' for the reaction near thermal ener-
206 1
BOHME ET AL.
gies (0.1 to 0.2 eV) of C3H3+ derived by electron impact on
C3H7Br (25):
+
(321 C 3 ~ 3 + N + H2C3N+
+H
Data for hydrogenation reactions of type [8] is also sparse. SIFT
experiments have shown that C3N+ rapidly hydrogenates at
300 K according to reaction [33] with a rate constant of 9.1 x
There is still little laboratory evidence for Scheme D in which
nitrogen entrainment by slightly hydrogenated hydrocarbon
ions occurs through reaction with nitrogen atoms. Nevertheless
this route remains an attractive option unless hydrogenation
reactions with HZare required to build up the hydrogen content
before neutralization to form the cyanopolyacetylene. The
efficiency of hydrogenation appears variable and unpredictable.
New results are presented for the hydrogenation of oddnumbered carbon ions which leads to the slightly hydrogenated
hydrocarbon ions which can initiate Scheme D.
Acknowledgment
10-lo cm3 molecule-' s-' at ca. 0.3 Torr in helium buffer gas
(21). The further hydrogenation of HC3Nf is much slower at
300 K and competes with a dissociative channel in the manner
expressed by reaction [34] (21, 26):
OS(0 24)
C2H2++ HCN
Rate constants at 300 K of 1.9 and 7 X lo-'' cm3 molecule-' s-' have been reported for this reaction (21, 26). The
temperature dependence of this slow reaction is not known but it
is conceivable that the rate constant for this reaction increases as
the temperature drops in analogy with the reaction of NH3+ with
H2 (17).
Conclusions
I
I
'
Some progress has been made in this study and others carried
out over the past few years in the characterization of the
ion/molecule reactions which constitute the four reaction
schemes proposed for the gas-phase synthesis of interstellar
cyanopolyacetylenes. Substantial gaps still exist, but there is
now sufficient data to allow some discrimination between the
four schemes.
The polyacetylenic ions required for Scheme A are readily
generated by sequential reactions involving C2 and C4 units.
But the entrainment of nitrogen through an eventual bimolecular
reaction with HCN appears to succeed only with C2H2+ and
thus only for the formation of cyanoacetylene. The four-carbon
ions do not react with HCN in a bimolecular fashion at 300 K;
C4H2+ was observed to react with HCN in a rapid termolecular
association reaction. The success of this scheme must therefore
rest on the reactions with CN for which there is no information.
Scheme B relies on the build-up of cyanopolyacetylenes
by their repeated reaction with acetylenic ions. However,
measurements with cyanoacetylene suggest that this growth
may be short-circuited by proton transfer or by elimination
of HCN.
Growth by repeated addition of single carbon units to
cyanopolyacetylenes through sequential reactions with C + ions
and CH, neutrals as described in Scheme C appears to be quite
feasible. Results are presented for the critical reaction of C4N+
with CH4 which provides the bridge from cyanoacetylene to
cyanodiacetylene. The scheme is projected to higher members
of the cyanopolyacetylenes and is attractive especially in vlew
of the chemical feedback that is possible. More information is
required for the higher members of the homologous series.
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support.
1. W. M. IRVINE,F. P. SCHLOERB,
A. HJALMARSON,
and E.
HERBST.
In Protostars and Planets 11. Edited by D. C. Black and
M. S. Mathews. The University of Arizona Press, Tucson,
Arizona. 1985. p. 579.
and B. KHARE.
Nature, 277, 102 (1979).
2. C. SAGAN
and E. ANDERS.
Top. Curr. Chem. 99, 1 (198 1).
3. R. HAYATSU
and C. M. WALMSLEY.
Astrophys. Space Sci.
4. G. WINNEWISSER
65, 83 (1979).
and D. K. BOHME.
Astrophys. J. 232, 740 (1979).
5. H. I. SCHIFF
6. R. C. WOODS.In Molecular ions. Edited by J . Berkowitz and
K. 0.Groeneveld. Plenum Publishing Corporation. 1983. p. 5 11.
Astrophys. J. 272, 579 (1983).
7. H. SUZUKI.
Astrophys. J. Suppl. Ser. 53, 41 (1983).
8. E. HERBST.
and A. FREEMAN.
Mon. Not. R. astr. Soc. 207,
9. T. J. MILLAR
405 (1984).
10. A. B. RAKSITand D. K. BOHME.Int. J. Mass Spectrom. Ion
Processes, 55, 69 (1984).
G. D. VLACHOS,
D. K. BOHME,
andH. I. SCHIFF.
11. G. I. MACKAY,
Int. J. Mass Spectrom. Ion Phys. 36, 259 (1980).
In The handbook of preparative inorganic chemis12. 0. GLEMSER.
try. Edited b y G. Brauer. Academic Press, New York. 1963.
p. 658.
13. A. B. RAKSIT
and D. K. BOHME.
Int. J. Mass Spectrom. Ion Phys.
49, 275 (1983).
and D. SMITH.J. Phys. B, 9, 1439 (1976).
14. N. G. ADAMS
W. T. HUNTRESS,
and M. J. MCEWAN.
J. Phys.
15. V. G. ANICICH,
Chem. 90, 2446 (1986).
16. S. DHEANDHANOO,
L. FORTE,
A. FOX,andD. K. BOHME.
Can. J.
Chem. 64, 641 (1986).
J. Phys. Chem. 85, 1091 (1981).
17. F. W. BRILLand J. R. EYLER.
G. A. BLAKE,J. K. KIM,M. J . MCEWAN,
and
18. V. G. ANICICH,
J. Phys. Chem. 88,4608 (1984).
W. T. HUNTRESS.
L. W. SIECK,R. METZ, S. G. LIAS,and J.
19. T. J. BUCKLEY,
LIEBMAN.
Int. J. Mass Spectrom. Ion Processes, 65, 181 (1985).
and D. K. BOHME.
Can. J. Chem. 63,854 (1985).
20. A. B. RAKSIT
C. G. FREEMAN,
M. J. MCEWAN,
S. C. SMITH,
21. J. S. KNIGHT,
N. G. ADAMS,and D. SMITH.Mon. Not. R. astr. Soc. 219,
89 (1986).
22. D. K. BOHME,
S. WLODEK,
and A. B. RAKSIT.Can. J. Chem.
To be published.
23. E. HERBST,
N. G. ADAMS,and D. SMITH.Astrophys. J. 269,
329 (1983).
A. B. RAKSIT,and H. I. SCHIFF.Chem. Phys.
24. D. K. BOHME,
Lett. 93, 592 (1982).
H. VILLINGER,
W. LINDINGER,
and E. E. FERGU25. W. FEDERER,
SON.Chem. Phys. Lett. 123, 12 (1986).
S. DHEANDHANOO,
and D. K. BOHME.
26. A. Fox, A. B. RAKSIT,
Can. J. Chem. 64, 399 (1986).
27. J. A. LUINEand G. H. DUNN.Astrophys. J. 299, L67 (1985).